1. Field of the Invention
This invention relates to apparatus for converting data between a digital form and an analog form. More particularly, the invention relates to apparatus which provides such conversion instantaneously and on a monotonic basis and with minimal differential and integral errors.
Various types of equipment receive information in analog form. Such equipment includes process control equipment, measuring instruments, communications equipment and a wide variety of other equipments. Digital computers and data processing systems often receive input parameters in analog form from such equipment and convert these parameters to digital form for processing in the computer or the data processing equipment. After the analog information has been converted to digital information and has been processed, the output information from the digital computer or the data processing equipment is often converted to analog form. By converting the digital information to analog form, the user can assimilate the information in ways which would be difficult if the information remained in digital form.
A good example of the conversions discussed in the previous paragraph is in the recording and reproduction of music. The music is produced in analog form. It is converted to digital form by recently developed data processing techniques and is recorded on a medium such as a tape or a disc. When the music is to be reproduced, it is converted again to analog form because this is the form which is necessary to operate sound transducers to give meaning to the listener when he hears the music.
As digital computers and data processing equipment have become widespread throughout industry and the office and have even become common in the home, the need for inexpensive, simple and reliable apparatus for converting information between digital and analog forms has become of increasing concern. A considerable effort has been devoted over a period of several decades to provide for converting apparatus which is simple, inexpensive and reliable. In spite of such efforts, the converting apparatus now in use does not meet such criteria.
The converting apparatus now in use also has other problems of long standing. For example, the converting apparatus now in use may not be monotonic unless it is quite expensive and complex. By "monotonic" is meant that digital information of progressively increasing value is converted to analog information of progressively increasing value without any decrease in the analog value as the digital value progressively increases.
The converting apparatus now in use also has relatively high differential and integral nonlinearities unless the apparatus is quite expensive and complex. Integral nonlinearities result from errors produced in a conversion between digital and analog values over a wide range of such values Differential nonlinearities result in errors produced in a conversion between digital and analog values over a relatively narrow range of such values.
The converting apparatus now in general use also has a problem of major proportions. This results when the converter is operating near the mid-point of its full scale range, or near mid-scale, and digital values are increased incrementally by a single digit. For example, mid-scale problems result in the 12 bit converters now in use when a binary representation of 2047 is converted to a binary representation of 2048. This results from the fact that the binary representation of 2047 is represented by 011111111111 and a binary representation of 2048 is represented by 100000000000 where the least significant digit is at the right. Thus, at mid-scale, the value of every one of the binary digits changes. As all the binary values change, different weighting elements within the converter are selected and discontinuities may occur. These discontinuities may prevent the converter from being truly monotonic. This problem even exists in converters which are made quite complex in an attempt to overcome the problem.
In copending application Ser. No. 383,544 filed by Henry S. Katzenstein on June 1, 1982, for "Apparatus for Converting Data Between Analog and Digital Values" and assigned of record to the assignee of record of this application, apparatus is disclosed and claimed for converting between digital and analog values on a monotonic basis. The converter disclosed and claimed in application Ser. No. 383,544 has certain important advantages. For example, the converter provides an instantaneous conversion between digital and analog values on a monotonic basis for any digital value. The converter provides this conversion with relatively low differential and integral nonlinearities. The converter is quite simple in construction and is reliable in operation.
In copending application Ser. No. 553,041 filed by Henry S. Katzenstein on Nov. 18, 1983, for "Apparatus for Converting Data Between Digital and Analog Values" and assigned of record to the assignee of record in this application, a converter is also disclosed and claimed for converting between digital and analog values. The converter of application Ser. No. 553,041 constitutes an improvement of the converter of application Ser. No. 383,544, at least for converting information at relatively low frequencies. The converter of application Ser. No. 553,041 is similar to the converter of application Ser. No. 383,544 except that it employs energy storage members such as electrical capacitors to provide a conversion between the digital and analog values. The converter of application Ser. No. 553,041 is also advantageous in that the number of binary bits capable of being converted on an integrated circuit chip is enhanced and the energy storage members can be easily formed on the chip. The use of such energy storage members is also advantageous because they can be formed with minimal differences between them and because they are relatively stable with changes in temperature.
This invention also relates to apparatus for converting between digital and analog values. This invention can be considered to provide a converter distinct from, and advantageous over, those disclosed and claimed in applications Ser. Nos. 383,544 and 553,041. However, in one embodiment of the apparatus constituting this invention, such apparatus can employ a converter similar to those disclosed in applications Ser. Nos. 383,544 and 553,041, and particularly the converter disclosed and claimed in application Ser. No. 553,041. With such a converter, the apparatus of this invention also employs other stages for facilitating the conversion between the digital and analog values. The apparatus of this invention provides a conversion between the digital and analog values with even greater monoticity, and with even less integral and differential nonlinearities, than the converters of applications Ser. Nos. 383,544 and 553,041.
In one embodiment of the invention, a first matrix relationship is defined by a plurality of switches which are operative in first and second states in accordance with the pattern of binary signals introduced to the switches. An activating line is connected to the matrix relationship to activate energy storage members, such as capacitors, connected to the matrix relationship. The number of energy storage members energized by the activating line at each instant to store energy is related to the value coded by the logic levels of the binary signals introduced to the matrix relationship. For increasing digital values, the storage members previously energized in the plurality by the activating line continue to be energized and additional storage members in the plurality are energized as the digital values increase.
An interpolating line is provided in the first matrix relationship in addition to the activating line. The interpolating line receives a voltage related to the binary value coded by the logic levels of the binary signals of relatively low binary significance. This voltage may be produced by a second matrix relationship of conventional construction. This voltage is introduced through the interpolating line to a particular one of the storage members in the plurality. The particular storage member is the one to be connected next to the activating line for increasing digital values.
An output signal is produced corresponding to the cumulative value of the energy stored in the storage members in the plurality by the activating line and the energy stored in the particular storage member by the interpolating line. The output signal may be produced by an integrating amplifier connected to the storage members in the plurality.